1. Field of the Invention
The invention relates to a device for non-thermal excitation and ionization of vapors and gases. The non-thermal excitation of vapors and gases is widely used in industrial applications for the cracking and decomposition as well as for the synthesis of simple and highly molecular compounds of organic and inorganic nature.
2. Description of the Prior Art
Because they can be controlled easily, it is preferred to use electric fields and discharges for excitation. Discharges are electric currents through a gas. Based on their current-voltage characteristics, they are divided into different types, such as Townsend (independent or induced dark discharges), corona or barrier discharges, normal and abnormal glow, spark, and arc discharges.
Technically, (cold plasma) corona and glow discharges are used for weak excitations up to multi stage excitations. Spark and arc discharges cannot be used in non-thermal methods.
So far known excitation devices can be divided into two basic types: devices with plate-like, flat electrodes and devices with concentric, tube-like electrodes.
The excitation, the partial or the complete ionization of the gazes often leads to the formation of clusters, which are combining into aggregates and aerosols due to collisions and finally form larger droplets. Experience shows that condensation on the discharge surfaces also in humid gases (decrease of the dew-point). Such condensates, which are deposited on the electrodes or their coatings, respectively, can strongly influence the current transition by local modification of the electrical resistance. In this way a local increase of the conductivity, e.g. due to water droplets, can lead to local spark discharges, break through and even arc discharges. This leads to a damage of the barriers or the coatings, respectively, of the electrodes, to increased current consumption and to an undesired heating.
Depending on the gas composition a polymerization can occur as well, resulting in a fog of polymers, which is deposited on the electrodes or the dielectric and modifies the discharge conditions. Such effects are e.g. known from the treatment of styrene or ethylene oxide containing vapors. During excitation of such gases, the polymerization of the monomers is started and, after a short time, the barrier material and/or the electrodes are coated by a polymer layer. As a consequence, an additional isolating layer is formed and the discharges loose their intensity.